1. Background Field
Embodiments of the subject matter described herein are related generally to position determination based on signals from wireless transmitters, and more particularly to the distribution of almanac data for wireless transmitters for position determination.
2. Relevant Background
Position location systems for mobile devices in satellite positioning challenged/denied areas may use positioning based on signals from local wireless transmitters, such as cell towers or base stations for cellular wireless networks, or WLAN access points, femtocells, etc. However, positioning using wireless transmitters conventionally requires an assistance server that can provide an almanac that contains the locations of the wireless transmitters.
Conventionally, almanac databases are built using crowdsourcing, wardriving and/or a combination of both. Both crowdsourcing and wardriving require that multiple mobile devices collect data related to the location of the wireless transmitters and submit the collected data to a centrally located server. The server combines the collected data from multiple users to generate the almanac database that includes the locations of the wireless transmitters. In order to use wireless transmitters for positioning, a mobile device accesses the almanac database through the server, or different servers, and uses the locations for visible wireless transmitters to determine the position of the mobile device. As the almanac database may be large, a mobile device typically downloads only the relevant regional content, sometimes referred to as “tiles,” from the server.
FIG. 1, by way of example, illustrates a system 10 in which a mobile device 12 provides collected wireless transmitter data to a server 14 and accesses the data through content servers 16. The server 14 aggregates crowdsourced and/or wardriving information from multiple mobile devices to create the almanac database for wireless transmitters that includes the locations of the wireless transmitters. When the mobile device 12 uses local wireless transmitters for positioning, the mobile device 12 contacts one or more content servers 16 (which may include the server 14) to download relevant regional content based on visible wireless transmitters. After downloading the regional content from the content servers 16, the mobile device 12 can obtain the locations of visible wireless transmitters to be used in determining the position of the mobile device 12.
When a position fix is required by the mobile device 12, access to the necessary regional content is required in real time. Thus, the mobile device 12 must have data connectivity to access the content servers 16 to acquire the regional content, or the mobile device 12 must have previously downloaded the regional content. Data connectivity, however, cannot always be assured. Moreover, at the time of a position fix calculation, the mobile device 12 only requires the locations of visible wireless transmitters, i.e., wireless transmitters that the mobile device 12 is actually wirelessly observing. The regional content, however, typically includes multiple nearby wireless transmitters that are not visible to the mobile device 12, which is downloaded onto the mobile device 12 along with the visible wireless transmitters. Accordingly, downloading regional content is a waste of bandwidth as only the data related to visible wireless transmitters is needed. Additionally, downloading regional content by mobile devices before a position fix is required is also a waste of bandwidth as the position fix may not be required at all.